JP2003-166508A discloses a hydraulic cylinder in which a first passage and a second passage are formed in a piston rod.
In this hydraulic cylinder, an interior of a cylinder tube into which the piston rod is inserted is partitioned into a head-side chamber and a bottom-side chamber by a piston that is linked to the piston rod. When the hydraulic cylinder is operated so as to extend, pressurized working oil from a hydraulic power source is supplied to the bottom-side chamber through the first passage, and the working oil in the head-side chamber is returned to a tank of the hydraulic power source through the second passage. When the hydraulic cylinder is operated so as to contract, the pressurized working oil from the hydraulic power source is supplied to the head-side chamber through the second passage, and the working oil in the bottom-side chamber is returned to the tank of the hydraulic power source through the first passage.
The first passage and the second passage are defined by two axial holes respectively formed in the solid piston rod.
The first passage communicates the hydraulic power source with the bottom-side chamber. On a first end of the axial hole defining the first passage, a first port that is connected to the hydraulic power source through a pipe is formed. A second end of the axial hole defining the first passage opens to the bottom-side chamber.
The second passage communicates the hydraulic power source with the head-side chamber. On a first end of the axial hole defining the second passage, a second port that is connected to the hydraulic power source through a pipe is formed. In a middle part of the axial hole defining the second passage, a communicating hole that extends in the radial direction of the piston rod and opens to the head-side chamber is formed.
On the axial hole defining the second passage, an assembly hole that extends in the radial direction of the piston rod from a vicinity of an open end of the bottom-side chamber is formed. A columnar closing member (a plug) is plugged in the assembly hole, and communication between the head-side chamber and the bottom-side chamber is blocked by the closing member.